Controlled or slow release fertilizers are fertilizers that include a plant nutrient in a form that delays availability of that nutrient for plant uptake and use after application. That is, a controlled or slow release fertilizer extends availability of a constituent plant nutrient beyond that of a rapidly available nutrient fertilizer. Rapidly available nutrient fertilizers that are sources of nitrogen include ammonium nitrate, urea, ammonium phosphate, and/or potassium chloride (The Association of American Plant Food Control Officials' Publication 1985).
The delay of initial availability or extended time of continued availability that are possible with controlled or slow release fertilizers occur by a variety of mechanisms. One such mechanism relates to controlled water solubility of the pertinent plant nutrient. Control of water solubility can be effected by employing, for example, semipermeable coatings or occlusion. Control of water solubility may also be effected by relying on the inherent water insolubility of certain polymers, natural nitrogenous organics, protein materials, or other chemical forms. Further means for controlling water solubility include relying on the slow hydrolysis of water soluble low molecular weight compounds (The Association of American Plant Food Control Officials' Publication 1985).
Rapidly available nutrient fertilizers provide a large amount of a plant nutrient at one time. This large dose of plant nutrient can lead to large flushes of growth immediately following application, which can be undesirable. This effect can be mitigated by applying small doses of the fertilizer sequentially over time. However, adopting such a strategy involves multiple applications, increasing the time and cost associated with growing. To address this difficulty, controlled or slow release fertilizers are used in conjunction with rapidly available nutrient fertilizers to allow for fewer applications, while achieving an even plant nutrient release rate. This strategy decreases the occurrence of flushes of growth, as mentioned above, and reduces the environmental effects associated with leaching that can occur when a single, large application of rapidly available nutrient fertilizer is employed. Controlled or slow release fertilizers, when used correctly, allow for the single application of fertilizer with the effect of multiple small applications of a rapidly available nutrient fertilizer.
Struvite, also known as magnesium ammonium phosphate hexahydrate, is a compound that is commonly found as a precipitate on pipes. Much investigation has been done on struvite to avoid its accumulation in pipes, but there has also been work relating to enhancing struvite precipitation in waste water to decrease phosphorus load. The struvite precipitate can then be dried and used as a phosphorus rich fertilizer source. Struvite is a desirable fertilizer because of its low solubility in water, which confers a controlled or slow release nature to the fertilizer.
Various methods of synthesizing magnesium ammonium phosphate are known. For example, U.S. Pat. No. 3,320,048 discloses a method in which either aqueous ammonia or liquid anhydrous ammonia is mixed with magnesium hydroxide and aqueous phosphoric acid to obtain magnesium ammonium phosphate granules, which are, in turn, dried and screened. U.S. Pat. No. 6,776,816 discloses a method in which animal manure is mixed with a magnesium-rich compound and an enzyme, such as a urease or uricase, to precipitate magnesium ammonium phosphate.